Satellites and space probes generally include photovoltaic generators intended to supply power to the onboard equipments and to charge the batteries that supply power during eclipses.
Electrical energy generation systems for space applications are conventionally designed so that the photovoltaic generators operate at a predetermined point on their voltage-current characteristics. This solution has the advantage of simplicity, but cannot maximize the power generated. In fact, it is known that photovoltaic generators have an optimum operating point, known as the maximum power point MPP=(VMPP, IMPP), that is highly dependent on the temperature and the illumination of said generators and varies over time as they age.
For this reason, it is known to connect photovoltaic generators to their load (a power distribution busbar or an energy storage device such as a battery) via DC voltage converters presenting transconductance that can be varied automatically in order to “track” the optimum operating point (this is known as maximum power point tracking (MPPT)).
That technique is increasingly used in space applications.
For example, the Rosetta probe of the European Space Agency (ESA) is intended to operate in widely varying conditions of illumination and temperature (reduction of illumination by a factor of 25 accompanied by a temperature drop of 360° C. during the mission), making it necessary to use a MPPT controller. The Rosetta power system has been used again for two other ESA missions, Mars Express and Venus Express.
It is generally very important for an electrical energy generation system for space applications to have a modular structure, with good segregation between modules. Here “segregation” refers to the fact that failure of one element must not propagate to others. More generally, good fault tolerance is looked for; for example, it is desirable for an isolated fault in a power control device (switch, regulator, converter, etc.) not to cause any loss of power.
Prior art MPPT (maximum power point tracking) electrical energy generation systems do not have satisfactory segregation and fault tolerance properties. Moreover, it is difficult in such systems to verify reliably that each module is operating correctly after it has been integrated.
For example, in the system used in the Rosetta, Mars Express, and Venus Express probes, segregation between the individual photovoltaic generators constituting the solar panels was sacrificed in the search for very high energy efficiency.
The document FR 2 885 237 describes an MPPT electrical energy generation system that segregates the individual generators and it is envisaged to use them in the Bepi-Colombo probe. However, the fault tolerance of that system is not entirely satisfactory because an isolated fault can cause the loss of an entire section of a solar panel.
In contrast, the power regulators commonly used by the European space industry achieve very good segregation of the generator modules and entirely satisfactory resistance to isolated faults. Those power control systems are essentially the sequential switching shunt regulator (S3R) described in the paper “The Sequential Switching Shunt Regulator S3R” by D. O'Sullivan and A. Weinberg, Proceedings of the Third ESTEC Spacecraft Power Conditioning Seminar, Noordwijk, Netherlands, 21-23 Sep. 1977, and the sequential switching switchover shunt regulator (S4R) described in the document FR 2 785 103.
However, those regulators do not implement photovoltaic generator maximum power point tracking.